analysis of clifton springs long jetty construction...
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Analysis of Clifton Springs Long Jetty Construction Methods
Flinders University Maritime Archaeology Field School
Portarlington 2008
Author
Anthony Mansfield
Acknowledgements
The author would like to thank his team supervisor, Jun Kimura, for his encouragement during the field school, and Dr Sam Turner of the St Augustine Lighthouse Museum for his questions regarding the structural detail of the jetty
which led to this particular project.
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Introduction This project is submitted as part of the assessment for the Flinders University
Maritime Archaeology Field School of February 2008 held at Portarlington, Victoria.
The Clifton Springs Long Jetty is an old jetty which has mostly collapsed. The
seabed around the jetty is littered with debris from the structure of the jetty.
Determining which part of the jetty the debris is from can be difficult if the original
structure is unknown. This project aims to assemble, from the observed jetty
remnants, the method of the jetty’s construction and thus provide a guide to the
original configuration of debris from the jetty.
History The jetty was constructed sometime between 1850 and 1888 to allow bay steamers
to transfer passengers to the Clifton Springs Baths. The jetty was used until 1920
and by 1925 had fallen into disrepair. After this time there is no records found of its
use by any commercial or government body. The jetty is today little more than a
long row of piles, some of which have completely fallen, and some remaining span
structure.
Aims This project has two main aims.
The first is to identify the method of construction of the jetty deck. This will allow
the correct identification of debris on the seabed around the jetty. A future
extension of this project would be to compare the construction of this jetty with
other jetty’s of the era to identify local variations.
The second aim is to identify non conformances and failures in the debris. Non
conformances would be items of debris which do not conform to the identified deck
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construction. Failures are the areas of the jetty structure which have failed in a
similar method and may have a common cause related to the method of
construction. The non conformances and failures will be briefly discussed.
A possible extension of this project would be to compare the construction methods of
this jetty with other jetties of the era which may shed light on questions such as
when the jetty was actually built and what the builders anticipated its service life
would be.
Significance The construction methods used demonstrate both the technology and materials
available at the time and the expected life of the jetty. These two aspects provide
information on the actual and expected activity at the site.
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Method The method adopted was to
measure the remnant jetty
structure still standing and
partially displaying
construction methods. By
assembling information from
along the remnant jetty a
composite frame system
could be developed.
The jetty was judged, upon closer inspection, to be too unsafe to climb on to get the
required measurements. A series of photographs were therefore taken with scale
bars. These photographs were then manipulated in Photoworks to produce the
dimensions of the remaining structure.
The seaward end of the jetty was photographed on the 10th Feb and the shore end
on the 15th Feb.
Dimensions and Descriptors While the scale bar and the initial measurements were SI units, the jetty was
originally designed and built in the era of imperial measurement. Thus the
measurements taken have been generally presented in imperial units as this provides
a more logical interpretation of the data.
In order to differentiate between the various different structural elements a naming
convention has been used. Lateral beams (across the jetty) are referred to as
bearers and longitudinal beams (along the jetty) as joists.
Figure 1 - Scale Bar Method
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Results
Part 1 – General Construction Method The jetty is a simple wooden construction. It is 700m long and 4m wide on the
deck. For the majority of its length is uses sets of three 12” diameter wooden piles
with wooden cross beams for each support span. The combination of piles and
beams in these support spans are the bulk of the recognisable debris on the seabed
and are thus the main focus of this project.
Figure 2 - General Construction
Piles The centre pile is vertical and the outer two piles lean inwards at an angle of
between 6 and 9 degrees. The piles are wooden (species not identified, anticipate a
locally sourced supply) and generally at least 12” diameter. Majority of wastage in
the remaining piles has occurred at the water line.
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Cross Beams (Bearers) Each set of three piles is sandwiched between two horizontal bearers of 12”x4” by
14’2” (min). Each bearer is rebated into the piles to a maximum depth of 2” and
secured by 1” iron through-bolts.
Figure 3 - Support Span Plan View
Longitudinal Beams (Joists) Resting on top of the bearers are four joists of 12”x4” by at least 14’6” (distance
over adjacent support spans) although these joists usually cover more than one
span. These joists are oriented with the long edge (12”) vertical. This provides the
strongest support for vertical deck loading.
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Figure 4 - Support Span Side Elevation
Joist Connections Two of the joists are secured outboard of the outer piles and two are secured on the
bearers part way between the mid and outer piles.
The two outermost joists are secured to the outer piles by a single horizontal
through-bolt and to each of the bearers by a vertical through-bolt. Where two outer
joists connect end to end at the outer pile they use a stepped butt join with two
horizontal bolts through the pile and the normal vertical bolts through the bearers.
Note that the step in the joist does not begin until the inner edge of the bearer.
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Figure 5 - Joist End Connection Detail - Plan View
The two inner joists do not run along side any piles so these are simply secured by a
vertical bolt through each bearer. Where two inner joists connect end to end the
join has been placed between the bearers and the same bolting arrangement used.
In this instance there is no direct connection between the ends of the joists.
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Figure 6 - Outer Joist End Connection Detail
Bolts The bolts are 1” diameter and lengths varying from 16” to 26”. Due to the minor
dimensional variations in the pile spacings and beams it is likely that the holes for
these bolts were bored in situ. An interesting technology question is how these
holes were bored, considering that some are up to 24” long through two beams laid
edge to edge and still bored accurately enough that they did not break out of the
beams’ sides.
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Figure 7 - Through-bolt nut detail
The bolts use square heads on one end and a thick square washer secured with a
square nut on the other end. Square nuts were used as they were simple to
manufacture and provided a large bearing surface for wrenches. This was important
if the nut material was not particularly strong, eg iron rather than steel.
These bolts may have been painted or tarred after installation to slow corrosion
however there was no evidence of this detected during this project.
Decking It is surmised that fairly substantial decking was laid over the joists however there is
no evidence remaining on the jetty of the sizes of this decking. The decking would
have had to support groups of people (loading of 250kg/m2 plus) and possibly
general cargo (loading unknown) over a span between joists of 4’4” so whatever was
used had to be quite strong.
The decking would have also assisted in tying together the joists and provided a
great deal of horizontal shear strength for the jetty. The loss of the decking, either
by storm damage or salvage, would have seriously weakened the remaining jetty
structure.
Deck Fixing An artefact recovered during the jetty excavation was an encrusted square section
iron spike. It is unknown where this spike originally came from but it may have been
a deck fixing spike. Without closer inspection of the remaining jetty structure this is
purely conjecture.
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Figure 8 - Spike encrustation showing square cross section.
Overall Strength Design The jetty had to resist a large number of directional forces. Some of these are:
Lateral wave loadings on the piles
Lateral ship loadings on the piles
Vertical (down) passenger and cargo loadings on the decking and support
structure
Vertical (up) wave slap loadings on the decking
The worst of these would have been the lateral loadings which would have been
resisted by the seating of the piles in the seabed and, to a lessor extent, the bolted
connections. The usual span design did not provide much lateral resistance,
generally acting as a pin jointed structure. The designers of the jetty did recognise
this however and addressed it with special structure, discussed in Part 2.
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Part 2 – Non conformances and failures
Cross Braced Spans Several support spans have been observed on the seabed which do not follow the
structure described in Part 1. These spans have, in addition to the two bearers, at
least one additional horizontal bearer around 4’10” (1.5 metres) lower on the piles
and a cross beam running diagonally across the span between these upper and lower
bearers. It could be reasonably assumed, based upon the overall structural design,
that the lower bearers and diagonal beams were also doubled and sandwiched the
piles.
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Figure 9 - Cross Braced Span (taken from seabed site plan)
These spans with diagonal bracing would have provided extremely good lateral
strength for the jetty. As only two of these have been identified in the seabed debris
it may be that these extra strength spans were only placed either at high load areas
(perhaps where ships would come alongside the jetty) or at regular intervals along
the outer end of the jetty.
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Two Pile Spans The shore end of the jetty dispenses with the mid pile for at least 50 metres out
from the beach.
This end of the jetty did not have to be as strong as the outer end and so it is likely
that the designer dispensed with the mid pile for economy reasons.
Bearer and Joist Breakage In both the seabed debris and the remnant structure it has been observed that the
joists and bearers tend to break in regular locations [see Figure 9 - Cross Braced
Span (taken from seabed site plan)]. These locations correlate to the locations of
the through bolts which join the bearers and joists. These bolted connections, while
providing overall strength for the jetty, have produced a weak spot in the beams.
During the general structural failure of the jetty these weak spots have regularly
failed, producing the common beam and joist configurations seen on the seabed.
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Conclusions The basic arrangements of structure described in this project is sufficient to identify
and explain the use of most items of structural debris found on the seabed during
the site survey. The project therefore has achieved its basic aims.